In the piece, Buffett argues that bonds "should come with a warning label" right now, because interest rates aren't high enough to make up for inflation and taxes.

Even though bonds, and other currency-based investments, have a reputation of being safe, "in truth they are among the most dangerous of assets."

Buffett says Berkshire holds U.S. Treasury bills only to make sure the company has quick access to cash if it is suddenly needed.

As for "non-productive" assets, most notably gold, Buffett argues their "owners are not inspired by what the asset itself can produce — it will remain lifeless forever — but rather by the belief that others will desire it even more avidly in the future."

While a rising price can encourage even skeptics to join in, "bubbles blown large enough inevitably pop."

Buffett writes that if all the gold in the world were combined into one cube, about 68 feet on each side, it would be worth $9.6 trillion at its current price of $1750 per ounce.

For that amount of money, "we could buy all U.S. cropland (400 million acres with output of about $200 billion annually), plus 16 Exxon Mobils (the world's most profitable company, one earning more than $40 billion annually.)"

Another trillion would be left over.

"A century from now the 400 million acres of farmland will have produced staggering amounts of corn, wheat, cotton, and other crops -- and will continue to produce that valuable bounty, whatever the currency may be. Exxon Mobil (XOM) will probably have delivered trillions of dollars in dividends to its owners and will also hold assets worth many more trillions (and, remember, you get 16 Exxons). The 170,000 tons of gold will be unchanged in size and still incapable of producing anything. You can fondle the cube, but it will not respond."

This is not a new position for Buffett. For several years now, he's favored stocks over bonds or gold.